Motor-driven compressors currently in use in commercial internal combustion engines usually consist of a brushless electric motor mounted in a aluminum housing and driving a centrifugal air compressor wheel within an enclosing compressor casing, as indicated in FIG. 1. Such motor-driven compressors are frequently located within the engine compartment of a vehicle where the surrounding environment is at a substantially elevated temperature. In the operation of such air compressors, the motor is energized from an external power source such as a battery, through an electronic controller which changes direct current from the battery to alternating current to produce a rotating magnetic field in the motor windings. The rotating field surrounding motor magnets mounted on the drive shaft generates torque that rotates the compressor wheel and shaft assembly. The compressor wheel induces air from the atmosphere or an air cleaner into the compressor air inlet and delivers it from the compressor casing at above-atmosphere pressure.
In many applications, such as in supercharging systems for internal combustion engines, small size is an advantage and compressor motors may be temperature sensitive. By a temperature sensitive motor, we mean a motor whose reliability may be at risk, or whose performance may be degraded, by the inability to dissipate heat generated during its operation and its resulting internal temperatures, for example, a small electric motor whose internal temperature, due to the heat generated by its winding losses and the inability to adequately dissipate the heat, may endanger the motor insulation.
Internal combustion engine supercharging systems and other applications desirably employ a compact air blower assembly provided by high speed brushless electric motor comprising a stator and a rotor and rotatable shaft carried by a pair of shaft bearings, with the rotatable shaft including a shaft extension extending outwardly from one of the shaft bearings. Such a compact air blower assembly can include a housing with a first portion carrying the electric motor and shaft bearings and a second portion forming a compressor enclosure about the shaft extension with a centrally located air inlet and a peripherally located compressed air outlet. The compressor enclosure encompasses a centrifugal compressor wheel which is formed by a solid back wall extending radially outwardly from a shaft-engaging hub and a plurality of air-moving blades projecting radially and axially outward from the back wall to edge portions that radiate within the compressor enclosure between the air inlet and the air outlet. The high speed brushless electric motor can rotate such a centrifugal blower wheel at high speeds to provide a flow of compressed air at its periphery.
In applications such as internal combustion engine supercharging systems, compromises in the selection and use of electric motors have been required because of hostile motor environments and the relative inefficiencies of small motors. Thus, in such applications the amount of compressed air that can be reliably produced on a continuous basis by current motor-driven compressors is frequently limited by the temperatures generated in the windings of the electric motor. Low-speed motors have employed some type of internal fan attached to the shaft to produce a flow of cooling air through the motor and around its internal components to limit their temperatures; however, with very high-speed brushless motors, the use of small internal fans imposes a significant and undesirable load on the electric motor, becomes rather difficult to implement and complicates the internal construction of the compressor housing assembly.